The Van Trump Report

NEW Research Indicates Waterhemp is Outpacing Herbicides

University of Illinois scientists have uncovered a somewhat terrifying skill possessed by some of agriculture’s most troublesome weed species – some strains of waterhemp are resistant to chemicals that have never even been applied to crops.

In a recent study, researchers examined how multiple-resistant waterhemp found in Illinois detoxifies chemicals designed to kill it by using syncarpic acid-3, aka SA3. The chemical has never been applied to corn crops because it actually does kill them. But the herbicide-resistant waterhemp in the U of I study was able to withstand SA3’s deadly effects.

SA3 is an early predecessor to Callisto, an HPPD inhibitor herbicide that is naturally tolerated by corn. This is because the corn plant has a detoxification system that neutralizes and cordons off the harmful chemical, explains Dean Riechers, professor in the University of Illinois department of crop sciences and co-author on a new study. Waterhemp and dozens of other weed species typically evolve to mimic corn’s detoxification systems. But since corn is not able to detox SA3, the fact that the waterhemp can raises alarms that even yet-to-be discovered herbicides of the future might not be effective.

This type of herbicide resistance is what’s known as “metabolic” resistance, sometimes referred to as “off-site” resistance. Historically, the type of resistance that farmers and researchers deal with most often is “target-site” resistance, according to Aaron Hager, Ph.D., associate professor of weed science at the University of Illinois.

“Every herbicide has a specific target site in a plant,” says Hager. “The herbicide has to bind to that target site and effectively shut it down in order to kill the weed.” Target-site mutations are often prompted by repeated applications of the same herbicide site of action. Metabolic resistance, on the other hand, involves more change in the plant than just at the target site alone. In this type of resistance, weeds develop the ability to rapidly metabolize, or break down, the herbicide before it can cause significant biotoxic effects to the weed.
Riechers and his team traced the biochemical reactions inside resistant waterhemp plants when treated with SA3 and found they had developed detoxification strategies that corn doesn’t have. “This is probably the first known example where waterhemp has evolved a detox mechanism that a crop doesn’t have. It’s using a completely different mechanism, adding to the complexity of controlling this weed,” said Riechers. This novel evolution of detoxification patterns is also potentially damaging for crop producers as well as herbicide manufacturers.

Most concerning is evidence that all weeds have the potential to develop varying degrees of metabolic resistance to all herbicides. Researchers say that while rotating herbicides and employing multiple modes of action is still an effective way to forestall target-site resistance, the strategies may not work with metabolic resistance.

Riechers recommends producers diversify their weed-fighting toolkit as much as possible. “You don’t want to just rely on one herbicide to control waterhemp, because if it is ineffective, then you’re in trouble. Remember to use crop rotation, tillage, and other sites of action,” he says. He also reminds producers to get out in the field twice a week or more to observe where the field is at in growth stage and identify any waterhemp, or other weeds. This will help assure you are applying the appropriate herbicides at the right time. Reichers also recommends exploring seed destruction options that destroy weed seeds that pass through the combine at harvest. (Sources: University of Illinois,, Farm Progress)

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